Revert tui code so it does not rely on in-process app server (#14899)

PR https://github.com/openai/codex/pull/14512 added an in-process app
server and started to wire up the tui to use it. We were originally
planning to modify the `tui` code in place, converting it to use the app
server a bit at a time using a hybrid adapter. We've since decided to
create an entirely new parallel `tui_app_server` implementation and do
the conversion all at once but retain the existing `tui` while we work
the bugs out of the new implementation.

This PR undoes the changes to the `tui` made in the PR #14512 and
restores the old initialization to its previous state. This allows us to
modify the `tui_app_server` without the risk of regressing the old `tui`
code. For example, we can start to remove support for all legacy core
events, like the ones that PR https://github.com/openai/codex/pull/14892
needed to ignore.

Testing:
* I manually verified that the old `tui` starts and shuts down without a
problem.

codex-rs/Cargo.lock

@@ -2500,7 +2500,6 @@ dependencies = [
  "chrono",
  "clap",
  "codex-ansi-escape",
- "codex-app-server-client",
  "codex-app-server-protocol",
  "codex-arg0",
  "codex-backend-client",